Solid state polymerization techniques for the preparation of polymers such as aromatic polyesters have been described; see, for example, Imai et al, U.S. Pat. Nos. 4,311,823 and 4,313,870; Kato et al, U.S. Pat. No. 4,327,205; Japanese Laid-Open Publications 79/46,287 and 79/46,291; and Belgian Patent No. No. 870,619. The processes claimed in the above patents consists in first preparing a prepolymer which is then solid state advanced to the required molecular weight. Thus Imai et al., U.S. Pat. No. 4,311,823 describe a process for preparing polyesters which utilizes three reaction vessels; wherein a dicarboxylic acid, an aromatic diol, and optionally hydroxybenzoic acid are reacted in the first vessel with an anhydride, such as acetic anhydride, to form the acetate monomers; acetic acid and excess of acetic anhydride are removed followed, if desired, by oligomerization of the mixture via heating at from about 150.degree. C. to about 280.degree. C.; the mixture is transferred to the second vessel where it is advanced to the prepolymer stage; depending on the temperature and on the degree of polymerization the resulting prepolymer is either a molten liquid which is cooled and pelletized prior to further reaction; or a polydispersed solid. The prepolymer is transferred to the third vessel where it is advanced in the solid state at elevated temperatures to yield the final polymer.
Processes wherein all of said three steps are performed in one reactor are described in, for example, U.S. Pat. Nos. 4,313,870 and 4,327,205. In both patents the prepolymer formation is conducted with application of sufficient shearing (or agitating) force to prevent polymer agglomeration, at a temperature and for a period of time such that the initially liquid reaction mixture is transformed into a polydispersed solid, which is then further reacted to high polymer below its sintering temperature.
The aforementioned patents disclose the preparation of a wide range of aromatic polyesters and other polymers. Examples, however, are limited to the rapidly crystallizing and highly crystalline p=oxybenzoyl based polyesters and to selected aromatic polyamides. No experimental conditions suitable for preparing the moderately crystalline Bisphenol-a polyarylates are suggested, nor is any technique disclosed for producing amorphous polymers although the patentees in some cases claim such polymers.
Polyarylates are polyesters derived from monomers consisting essentially of a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane, also identified as Bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. In addition, the polymers display outstanding UV stability. Polyarylates have good processibility which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative, which is then reacted with an aromatic dicarboxylic acid to form the polyarylate. Jackson et al, U.S. Pat. Nos. 3,684,766 and 3,780,148 describe a four step diacetate process for producing polyarylates. In the patented processes, in the first step a prepolymer is formed from, for example, a diacetate, such as Bisphenol-a diacetate, and an aromatic acid, in the presence of a catalyst. In the second step, the prepolymer so formed is comminuted into small particles. In the third step, these particles are contacted with a crystallizing agent to crystallize the polyester. This may be accomplished by placing the particles in a fluidized bed reactor and contacting them with the crystallizing agent in the reactor. The patent states that the crystallizing step is necessary because it allows to build up the molecular weight of the prepolymer in the next to a satisfactory level without fusing or sticking of the particles. The fourth step involves heating the crystallized bisphenol polyester in the presence of an inert gas, optionally in a fluidized bed, at a temperature below the melting point and for a period of time which is sufficient to achieve an inherent viscosity of at least 0.5 in the final polymer.
The crystallization step described in U.S. Pat. Nos. 3,684,766 and 3,780,148 can be a slow, time consuming and expensive operation.
Maruyama et al, U.S. Pat. No. 4,075,173 describe the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-a and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bisphenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2, of the patent: ##STR1##
This process is the diacetate process as described herein, or the "Acetate Process" as defined in the patent.
Column 2 of the patent states:
"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above mentioned literature." PA1 "On the other hand, the route (1), Acetate Process, is economically advantageous because the materials used are cheap and the operation is simple. For example, diacetate of Bisphenol-A, a monomer for Acetate Process, is synthesized by merely reacting acetic anhydride and Bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate Process, coloration and deterioration, are solved, Acetate Process will become the most superior process."
Further, column 3 of the patent states:
Thus, the skilled workers in the field of polyarylate chemistry realize that the existing processes for producing polyarylates have one or more deficiencies, and that a need exists to develop a viable diacetate process for producing polyarylates.
In U.S. Pat. No. 4,075,173, a copolyester was prepared by the diacetate process by a solid-state polymerization of low molecular weight prepolymers without using crystallizing agents. The patentees state that their Acetate Process is possible only when specific monomers are combined to form the prepolymer. These monomers are the diacetate of Bisphenol-A, terephthalic acid and/or isophthalic acid and an acetate of p-hydroxybenzoic acid. The prepolymer is then converted to the desired high molecular weight polymer by solid state polymerization. Specifically, in the process of said patent, a prepolymer is first prepared by condensing the acetate of p-hydroxybenzoic acid, Bisphenol-A diacetate with isophthalic and/or terephthalic acids at a temperature of from 200.degree. C. to 380.degree. C. for 1 to 10 hours. Catalysts capable of accelerating an ester exchange reaction can be used. The prepolymer is pulverized or pelletized and heated under reduced pressure or in an inert gas to convert it into a high molecular weight polymer.
Note, however, that the process of said U.S. Pat. No. 4,075,173 requires the use of p-hydroxybenzoic acid and produces polyarylates containing p-oxybenzoyl moieties. On the other hand, the processes described in U.S. Pat. Nos. 3,684,766 and 3,780,148 require the use of crystallizing agents in a crystallizing step.
Berger et al., U.S. Pat. No. 4,314,051 disclose a process for preparing polyarylates which comprises: (a) forming a polyarylate prepolymer having a reduced viscosity of from about 0.05 to about 0.4 dl/g by reacting at least one diester derivative of a dihydric phenol with at least one aromatic dicarboxylic acid in the presence of a solvent at a temperature of from about 250.degree. C. to 300.degree. C., and (b) heating the prepolymer so formed below is melting point to form a polyarylate having a reduced viscosity of from about 0.45 to about 1.2 dl/g. As indicated above the preparation of the polyarylate prepolymer is carried out in the presence of a solvent. The preferred solvents are a diphenyl ether compound as described in Berger et al., U.S. Pat. No. 4,294,956; and in Maresca et al., U.S. Pat. No. 4,296,232; a cycloaliphatic substituted aromatic or heteroaromatic compound as described in Berger et al., U.S. Pat. No. 4,294,957; and a halogenated and/or etherated substituted aromatic or heteroaromatic compound as described in Berger et al., U.S. Pat. No. 4,374,239. In this process solvent is desirable for minimizing color bodies in the polyarylate.
The process of U.S. Pat. No. 4,314,051 represents a significant improvement over the processes of U.S. Pat. Nos. 3,684,766 and 3,780,148 since it eliminates the need for a separate crystallization step of the prepolymer and does not require the use of an esterification catalyst. However, the use of a solvent entails the need for recovery and purification, which in turn increases the polymer cost.
Therefore, a need exists for an economical and practical diacetate process for producing high molecular weight polyarylates where color is immaterial such as in pigmented parts and printed parts.
It has now been discovered that certain Bisphenol-A polyarylates can be prepared by a bulk process.